An automobile assembly system includes a main line for assembling main parts to a vehicle body and a sub-line for assembling assemblies, such as doors, to be assembled with automobiles at the main line. In the main line, plural kinds of conveyors, such as an overhead conveyor and a friction conveyor are used in accordance with the kinds of main parts and assembling area of the vehicle body, and operators or workers are distributed into the corresponding processes. Meanwhile, the sub-line is provided adjacently to the main line. In the sub-line, plural kinds of conveyors are used in accordance with the assemblies, and operators or workers are distributed into the processes.
Generally, in the automobile assembly system, a vehicle body is transferred from a painting system with the doors and the like parts attached to the vehicle body. Accordingly, in consideration of the workability upon assembling various parts, doors and the like parts are removed firstly from the vehicle body carried along the main line. In the automobile assembly system, main parts are assembled to a vehicle body from which doors are removed, while the vehicle body without doors is carried along the main line, and subparts are assembled to the removed doors, while the doors are carried along the sub-line. For example, the sub-line for assembling door assemblies includes two lines, i.e. a conveyor for carrying a front right door and a rear right door with a single hanger, and a conveyor for carrying a front left door and a rear left door with a single hanger. In the case of a four-door vehicle, each hanger carries a front door and a rear door. Meanwhile, in the case of a two-door vehicle, each hanger carries a front door. In the automobile assembly system, the door assembly assembled in the sub-line is finally attached to the vehicle body in the main line.
In recent years, production of automobiles has been changed on demand of the user from “small types large-scale production” to “large types small-scale production”, and in the automobile assembly system, an assembly system is constituted such that a large number of types are assembled in the same single assembly line. Because the assembly method, the equipment, the specification, the number of parts and the like differ if the type of the vehicle is different, the number of processes for assembling the automobile is different for each type. For this reason, in the automobile assembly system applicable for many different kinds, the assembly line is constituted according to the automobile assembly method selected from many different kinds and with the maximum number of assembling processes, so as to enable assembly of all kinds of automobiles. Of course, if the automobile assembly system includes the main line and the sub-line, the sub-line is also constituted according to the automobile assembly method with the maximum number of assembling processes.
However, the automobile is classified from a small-sized vehicle to a large-sized vehicle, and the number of processes is considerably large in the type with the maximum number of processes, when compared with the type with the minimum number of processes. Therefore, in the automobile assembly system associated with plural kinds of types, the main line is constituted in accordance with the assembling processes of the type with the maximum number of assembling processes. As a result, the length of the line becomes longer and the number of operators distributed into each process increases. Therefore, when assembling the type with the minimum assembling processes in this automobile assembly system, operations are not performed in a large number of processes of the main line, leading to an excess of operators. In other words, when assembling various types of automobiles, man-hour deviations arise due to different types (car model). This leads to deteriorated assembling efficiency and increased space requirement of the whole system.
Further, the same car model includes various types, such as sedan, coupe and wagon, and the number of assembling parts or processes is different in each type. For example, with respect to the door, various types exist such as a two-door type vehicle, a four-door type vehicle and a rear slide-door type vehicle, and there also exists a type equipped with a tailgate. For example, FIG. 7 illustrates the number of assembling processes (for five different types) for assembling doors in a conventional automobile assembly system, in which A type has four doors (right and left front doors and right and left rear doors) and a tailgate, B type has three doors (right and left front doors and a tailgate), C type has five doors (right and left front doors, right and left rear doors and a tailgate), D type has right and left front doors, a left rear door and a tail gate, and E type has right and left front doors, a left slide door and a tailgate. Therefore, in the automobile assembly system associated with the above five different types, the sub-line for assembling door assemblies is constituted in accordance with the assembling processes (42 processes) of A type, which is the maximum number of processes, and operators are distributed into each process. The capacity of this sub-line is the maximum of 42 processes as the assembling capacity for assembling front door assemblies and rear door assemblies. However, since B type does not have right and left rear doors, D type has an exceptional left rear door, and E type has a slide door, these doors are not assembled as an assembly in the sub-line but are assembled as an assembly in the main line. For this reason, in the sub-line, an excess of processes arises for the processes for assembling the rear door assemblies, leading to a large number of excess operators distributed into these processes. Further, since the exceptional left rear door, the slide door and the tailgate are assembled as an assembly in the main line, the length of the main line becomes longer for the processes of assembling the slide door, the tailgate and the like, and the operators are distributed into each operation. In other words, since the number of doors or the number of processes for assembling each door assembly is different, man-hour deviations arise between each of the types. Further, in the case of multitypes, the type with a slide door and/or a tailgate should be taken into consideration, thereby leading to the increased number of assembling processes in the main line. As a result, the assembling efficiency is decreased and the whole system becomes larger in space requirement.